This invention relates to a control system having a microcomputer for controlling, for example, a vehicle safety device and the like.
As disclosed in Japanese Laid-Open (Kokai) Patent Application No. 193743/1990, a control system for an air bag (vehicle safety device) basically comprises an acceleration sensor, a microcomputer, and a drive circuit for supplying an electric current to a squib for the air bag. The microcomputer comprises a read only memory (ROM) for storing a program there in, a random access memory (RAM) for temporarily storing therein a detected data, a processed data, etc., a CPU for effecting calculation, etc.
The microcomputer is operated to effect an acceleration evaluation. In other words, upon inputting of an acceleration signal from the acceleration sensor, the microcomputer integrates i t, and compares this integral value with a threshold value. This integral value represents a change in vehicle speed, and increases in the decelerating direction upon a collision of the vehicle. When the integral value increases in the decelerating direction, and exceeds the threshold value, the microcomputer judges that a collision has occurred, and outputs a trigger signal, thereby expanding the air bag.
The above Japanese Laid-Open Patent Application does not disclose a failure judgment function of the control system. However, it is well known that a control system of this type requires various kinds of failure judgments. Therefore, it was demanded that such failure judgments are performed effectively and without interrupting a normal control operation.
Referring to other prior art, U.S. Pat. Nos. 4,873,452 and 4,950,914 disclose, respectively, a failure judgment function of an acceleration sensor as one of the component elements of the control system. German Patent Publication No. 3,820,534 discloses a failure judgment which is executed by a microcomputer after the microcomputer is reset by means of power-on resetting or by a watch dog timer.
Referring to a further prior art, Japanese Laid-Open Patent Application No. 202,233/1986 discloses a control system comprising a microcomputer having a CPU, a RAM, and a ROM, a watch dog timer, and power-on reset means (including a switch, a resistor, and a condenser). After resetting, the microcomputer executes a routine. First, it is judged whether or not the data stored in a specific area of the RAM is in agreement with an identification data stored in the ROM.
In the case where the microcomputer is reset by the power-on reset means, the data stored, before resetting, in all areas of the RAM including the specific area is already broken or lost. Therefore, it is judged "NO" and the program proceeds. In the next step, all areas of the RAM are cleared, and the identification data stored in the ROM is written into the specific area of the RAM. Thereafter, a normal controlling operation is effected.
When the microcomputer is reset by the watch dog timer because of runaway of the microcomputer, the data stored, before resetting, in all areas of the RAM including the specific area, is not broken but maintained. Therefore, it is judged "YES" and then the program proceeds to the normal controlling procedure, skipping the above described step. In this case, the data stored in all areas of the RAM is not cleared but maintained. Therefore, continuity of the controlling operation is maintained.
In a control system of another Japanese Laid-Open Patent Application No. 18,851/1989, a microcomputer distinguishes a power-on resetting from a resetting made by a watch dog timer based on an output of a D-type flip-flop. This control system has a backup power source as an additional part. The microcomputer also judges whether or nor it has been backed up by the backup power source. As in the above Japanese Laid-Open Patent Application No. 202,233/1986, the RAM is cleared, or otherwise the data stored in the RAM before the microcomputer is reset, is maintained without clearing the RAM, both in accordance with the kind of the resetting means.
The above Japanese Laid-Open Patent Application Nos. 02,233/1989 and 18,851/1989 do not disclose any kind of failure judgments of the control system.